The piston lock has wide application both as a primary locking device and as an actuator for another, primary locking apparatus. Some examples of their use as actuators in more complex structures appear in U.S. Pat. No. 4,590,777.
Piston locks, also called pin locks, are often packaged in barrel form. A cylindrical rotor is disposed in a cylindrical cavity in a larger diameter housing cylinder. The axis of the larger cylinder and its cavity are parallel but usually are spaced. One or more pairs of pistons are contained in a like number of aligned bores which extend normal to the housing and rotor axis and into both housing and rotor. Insertion of a proper key into a key slot of the rotor forces the pin to positions in which the parting plane of the pins of a pair occurs at the plane between the housing and rotor. In that condition the rotor is free to rotate in the housing and accomplish its locking and unlocking function. The pins of the housing are spring biased in the direction of the key slot. In past designs they have been free to fall out or spring out on removal of the rotor from the housing. Ordinarily that is not a problem. The cylinder need be removed from the housing only for servicing and to "change the lock" by interchanging or replacing pins to require a different key. However when it is a problem it can be a difficult one. It is not uncommon even for locksmiths to lose control of the pins and springs and once loose it takes skill and perseverence to replace them.